Field of the Background
The present invention relates to a method for flocculating latex particles in a latex, aqueous emulsion or microsuspension (hereinafter referred to simply as "latex") prepared by emulsion polymerization or microsuspension polymerization. The present invention also relates to a process for producing a thermoplastic resin.
Heretofore, a method of flocculating a latex followed by water removal has been industrially employed as a classical technique for obtaining solid rubber from a natural rubber latex and more recently in a process for producing ABS rubber. A flocculation technique used to be applied also to a vinyl chloride resin latex (i.e., a PVC emulsion).
Various methods have been known to destabilize an emulsion system of a latex, such as a method of adding a strong acid to the emulsion system, a method of adding a polyvalent metal salt to the system, a method of adding an organic flocculant such as methyl cellulose, carboxymethyl cellulose, polyacrylic acid, polyacryl amide, polyimide, polyamine or ammonium alginate to the system or a method of heating the emulsion system.
However, each of these methods has had its own demerits one way or another. There have been no method which is fully satisfactory for the flocculation of latex particles. For instance, when a salt is added to a vinyl chloride resin latex having a solid content of from 40 to 50%, the latex will solidify to form a bean curd-like or semi-solid mass, whereby the flowability will be completely lost and the subsequent treatment will be practically impossible. If the letex is diluted with water to have a solid content of from 15 to 20%, the flowability can be maintained even when the latex is solidified. However, there is no practical industrial method to remove water from the solidified product or a great amount heat energy used to be required to evaporate the water therefrom. Further, the solid aggregates obtained by such a drying step, are very hard and even when they are pulverized into fine powder, they can hardly be dispersed in a plasticizer and thus they can not practically be used as a plastisol. When the latex solidifies into a bean curd-like or semi-solid mass, the latex particles coagulate to form aggregates containing or occluding substantial amounts of water and the entire system forms a single solidified mass, whereby the flowability is completely lost. The solidified mass establishes a strong structure while occluding water therein, whereby centrifugal separation becomes practically impossible and the removal of water under reduced pressure likewise becomes impossible.
Under the circumstances, for the preparation of a thermoplastic resin such as a paste resin to be used for the preparation of a plastisol or an organosol (hereinafter referred to simply as "plastisol"), it has been common to employ a method in which a latex having an average particle size of at most about 3 .mu.m is prepared by emulsion polymerization or microsuspension polymerization and then the latex thereby obtained is subjected to spray drying. However, in order to use this method effectively, it is necessary to bring the solid content in the latex as high as possible. Otherwise, the cost for drying will be quite high. On the other hand, for a latex to be stable, it is necessary to maintain the solid content at a level of at most 45%. Otherwise, a great amount of aggregates are likely to form at the time of the polymerization, or a great amount of an additive such as an emulsifier is required, whereby a quality problem is likely to be led.
On the other hand, it is conceivable to minimize the cost of energy for drying by firstly flocculating a latex and then removing water from the flocculated latex by means of filtration or centrifugal separation to increase the particle concentration. However, if the latex is simply flocculated, the flocculated product will contain a substantial amount of water as mentioned above, whereby it is practically impossible to conduct the filtration or centrifugal separation of water and the flocculated product tends to be sticky or tends to have poor flowability. The industrial operation will thereby be very difficult.
Further, a paste resin must give a plastisol when dispersed in a plasticizer. However, a paste resin obtained by the flocculation with a polyvalent metal or a conventional floccurant, does not usually give a plastisol or at best gives a plastisol having an extremely high viscosity, whereby the commercial value as a paste resin is lost. Namely, the conventional flocculation methods are based on the idea of either deactivating the emulsifier in the latex, binding the latex particles to one another with polymer chains or removing the electric charges on the surfaces of the particles. In such flocculation methods, once the particles thereby obtained are dried, they are no longer capable of being dispersed in a plasticizer.